Neopentyl glycol (2,2-dimethyl-1,3-dihydroxypropane) is a white crystalline material widely used in the production of various industrial chemicals such as saturated and unsaturated polyesters, alkyl and polyurethane resins, powdered paints, synthetic lubricants, plasticizers, fiber processing agents and the like.
Neopentyl glycol is typically produced by an aldol condensation of isobutyraldehyde with formaldehyde, followed by hydrogenation over a metal catalyst, typically a nickel catalyst, as shown in the following Reaction Scheme: ##STR1##
U.S. Pat. No. 3,808,280 discloses a process for producing neopentyl glycol by carrying out the aldol condensation reaction in the presence of a tertiary amine catalyst. This method has, however, the problem that the tertiary amine catalyst reacts with organic acids, generated via Cannizzaro reaction of aldehydes during the condensation process, to form salts. Such salts deactivate the metal catalyst used in the subsequent hydrogenation and decompose the aldol condensation product during the process of distillation at a high temperature, thus lowering the yield of the desired neopentyl glycol.
Accordingly, when an amine catalyst is employed in the condensation process, it is required to remove most of the organic acid by-products from the condensation products.
On the other hand, U.S. Pat. No. 4,885,515 discloses a process for hydrogenating the aldol condensation product under a high-temperature, high-pressure condition using a copper chromite catalyst containing manganese, instead of a conventional nickel catalyst. However, this method is hampered by the problems of catalyst deactivation and an increased equipment cost due to the requirement of a severe reaction condition.
The product mixture obtained after the aldol condensation may contain such by-products as isobutyl aldoxane and neopentyl glycol isobutyrate, in addition to the above-mentioned organic acids. When directly introduced to the hydrogenation step, said by-products convert to isobutanol and trimethylpentanediol (2,2,4-trimethyl-1,3-pentanediol), respectively, the latter having a boiling point similar to that of neopentyl glycol. This compound is thus difficult to separate from neopentyl glycol.
Accordingly, in order to obtain neopentyl glycol in a highly pure form, it is required to remove the above-mentioned by-products, as well as the remaining aldol condensation catalyst and unreacted reactants, prior to the hydrogenation step.
Crude neopentyl glycol obtained after the hydrogenation step typically contains trimethylpentanediol and neopentyl glycol monohydroxypivalate as by-products, which are difficult to separate from neopentyl glycol by simple distillation, due to similar boiling points thereof.
Many methods have been attempted to purify the crude neopentyl glycol, and these include solvent extraction, vacuum distillation and crystallization methods. U.S. Pat. No. 2,895,996 discloses a process for the purification of crude neopentyl glycol by conducting saponification, followed by sublimation, which exploits the fact that neopentyl glycol is easily sublimable. However, this method is not commercializable due to the requirement that a low temperature must be maintained at the top of the sublimation apparatus.
Further, U.S. Pat. No. 4,935,555 suggests a method of distilling crude neopentyl glycol by using a membrane distillation apparatus under vacuum. However, this method requires expensive equipments, and gives a low yield of neopentyl glycol.